1. Field of Invention
The present invention relates to thermoplastic olefin compositions, and more particularly, to thermoplastic olefin compositions comprising a low-temperature impact modifying fluid.
2. Description of Related Art
Thermoplastic olefin (“TPO”) compositions typically comprise a blend of an olefinic homopolymer (e.g., polypropylene) with a rubbery copolymer of two different alpha olefins (e.g., an ethylene-octene copolymer). The rubbery copolymer improves the low temperature impact strength (toughness) of the TPO composition, but it also lowers the flexural modulus (stiffness) of the TPO composition. FIG. 1 shows an exemplary plot of the instrumented impact strength at −30° C. versus flexural modulus for polypropylene homopolymers having a 0.7 g/10 min melt flow index (“MFI”), 4.0 MFI, 12.0 MFI, and 30.0 MFI, respectively, that have been blended with 15% by weight and 30% by weight, respectively, of an ethylene-octene copolymer. The slope of the line defined by the data points for the 15% by weight and 30% by weight ethylene-octene copolymer loadings for each MFI illustrate the stiffness vs. impact tradeoff that occurs by adding the rubbery copolymer to the homopolymer. The tradeoff is affected by the MFI of the homopolymer, but it occurs at all MFI's.
The copolymer component of TPO compositions is relatively expensive, at least as compared to the homopolymer component. Accordingly, it would be highly desirable to minimize the copolymer component of TPO compositions while at the same time retaining the desired mechanical properties. This has heretofore been quite difficult.
One known method for reducing the copolymer content of a TPO composition while retaining desired low temperature impact strength is to add mineral oil to the composition. Oil containing TPO compositions, which are sometimes referred to as oil-extended rubber compositions, display minimal improvements in low temperature impact strength, but do so at a substantially greater loss in flexural modulus. FIG. 2 shows an exemplary plot of instrumented impact strength at −30° C. versus flexural modulus for 4 MFI polypropylene homopolymer blended with 17%, 21% and 25% by weight, respectively, of an ethylene-octene copolymer, and 0.1% to 8% by weight, respectively, of mineral oil. FIG. 2 shows that the relatively low gains in low temperature impact strength are obtained at a significant loss in flexural modulus.